1. Field of the Invention
The present invention relates to a superconducting quantum interference device (called "SQUID" in the specification), and more specifically to a novel SQUID having a superconducting current path constituted of an oxide superconductor material.
2. Description of Related Art
Of electronic devices utilizing the superconduction phenomenon, a SQUID is one of the devices most advanced in reduction into practice.
The SQUID is formed of a closed-loop superconducting current path having one or two weak links inserted thereto. A superconducting current flowing through the closed-loop superconducting current path cannot exceed a critical current of the weak link portion, and therefore, a current density in the closed-loop superconducting current path is extremely small. Accordingly, the momentum of cooper pairs existing in the superconducting current path is small, and on the other hand, the wavelength of a corresponding electron wave is extremely long. As a result, it can be regarded that the phase is the same in different portions within the superconducting current path. However, if a magnetic field is applied to this superconducting current path, a phase difference occurs in the superconducting current path. By utilizing this unique phenomenon, the SQUID can be used as a magnetic sensor having an extremely high sensitivity.
In fact, the SQUID has already actually been used not only in a fundamental measurement field for a magnetization meter, a NMR, a magnetic thermometer, etc. but also in a medical field for a magnetic cardiograph, an electroencephalograph, a magnetic tracer, etc., and in the field of earth science for a geomagnetism observation, an earthquake prediction, a resource prospecting, etc.
On the other hand, the superconducting material known in the prior art had become a superconductor only at an extremely low temperature not greater than a liquid helium temperature, and therefore, it had not been considered to practically utilize the superconducting material. However, since it was found in 1986 that compound oxide sintered material such as (La, Ba).sub.2 CuO.sub.4 or (La, Sr).sub.2 CuO.sub.4 are a superconductor material having a high critical temperature (Tc), it has been confirmed from one to another than a compound oxide such as a Y--Ba--Cu--O type compound oxide or a Bi--Ca--Sr--Cu--O type compound oxide shows a superconduction characteristics at an extremely high temperature. This material showing the superconduction characteristics at the high temperature permits to use an inexpensive liquid nitrogen for a cooling medium, application of superconduction technique has abruptly been put under study as an actual matter.
Therefore, if the oxide superconductor is used in the SQUID, it is expected to further promote spread of the SQUID which has been actually used in the above mentioned various fields. However, the SQUID actually formed of the oxide superconductor has internal noise generated by the SQUID itself, and therefore, can have only a substantially low sensitivity. When the SQUID is used as a sensor, it is possible to eliminate external noises by using a high degree of gradiometer, but it is very difficult to eliminate internal noises. Because of this, it is difficult to use the SQUID composed of an oxide superconductor, as a low-noise high-sensitivity sensor.